We demonstrate a method to manipulate the morphology of thin film semiconducting polymers. The manipulation is achieved by optical excitation of the polymer during roll-to-roll slot-die coating, providing a technique that is viable for large-scale production. Along with establishing the technique, the entire knowledge chain from fundamental insight in polymer physics and structure to application is presented. By correlating X-ray and neutron scattering techniques with density functional theory and molecular dynamics simulations of solvent evaporation, we successfully determine the packing of the polymers in the ground state and excited state. The findings are coupled with measurements of dynamical physical properties and solar cell device performance to pinpoint the structure-property relationship.

Different processing was applied to match energies below and above the excitation energy levels of P3HT in solution and in thin film [1]. Specifically, either no illumination (i.e., dark), or LEDs emitting red, green, or blue light illuminated the films while being coated. It is known from density functional theory (DFT) calculations [2] that, when exciting P3HT with visible light, i.e. going from the ground state to the excited state, the electron density changes to form double bonds between neighbouring thiophene rings. This change in the bonding pattern planarizes the polymer backbone. Consequently, the final morphology is affected when exciting the polymer during coating. To probe the morphological changes caused by the four treatments, we use Grazing-Incidence Wide-Angle X- ray Scattering (GIWAXS) on the coated films. To elucidate the physical origin of the changes, DFT and solvent evaporation molecular dynamics (MD) are used to simulate the effect of excitation on single polymer chains and thin films. The full picture of morphological changes is followed by a thorough investigation of the change of physical properties in the thin film. Here, the UV-VIS absorption spectra are analysed, followed by THz spectroscopy measuring the thin-film photo- conductivity. Finally, the consequence of light treating a complete OPV device (P3HT:O-IDTBR) is discussed.

The experimental and computational results demonstrate that exciting P3HT with visible light during deposition serves as a tool for manipulating the packing behaviour of P3HT and can be used to modify the final thin film morphology. It can be considered a new processing parameter for achieving the desired performance of organic thin films. Particularly, the capability to increase the out-of-plane mobility by light treatment can be used for transistor applications where directional mobility and patterning is essential [3]. Application in OPV devices will require an optimization process for the right crystal packing and domain size of donor and acceptor constituents. For the specific combination P3HT:O- IDTBR, the domain size decreases with light treatment and still results in a decrease in power conversion efficiency. Understanding how light treatment during fabrication influences the final morphology of a film can enable major improvements for specific materials systems or other technologies in flexible electronics and photovoltaics.

In conclusion, we report a method to manipulate the morphology of P3HT thin films through illumination with visible LED light during roll-to-roll slot-die coating. Optical polymer excitation temporarily constrains large sections of the chains into a planar geometry that is the minimum of the excited state potential energy surface. This structural effect is strong enough to affect the aggregation behaviour and, thereby, the final morphology of the P3HT film. The light-treated films are less crystalline overall, display a higher degree of face-on orientation, shorter conjugation length, and a change of the unit cell dimensions with less efficient packing. Consequently, the in-plane photoconductivity decreases and the out-of-plane conductivity increases drastically with light treatment.